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12/18/2014

   I've done this point in the recent past but not in this context. A more practical look at
Kirchoff's voltage rule. Perhaps application specific says it better. In the last post we looked at
getting and converting current which we know is part of the horsepower equation. Thing is it is,
according to Kirchoff, it is inverse and proportional. This would seem to suggest that whatever
we do converting current to torque has a proportional negative effect on RPM cancelling out
the work we've done. Take a breath.

      When Kirchoff wrote that rule there was an implied assumption. It assumed his audience
was aware that this rule holds IF there is a sufficient magnet flux contribution to the gap flux. It
also assumes zero dynamic timing advance. That's a new one, eh? Dynamic timing? I'll get
back to that in a minute or two.

      Sufficient magnet flux contribution is up first. This means the armature field strength is in
the knee of the magnets operating curve. Don't worry about what that is. Worry about what it
means. In simplest terms it means magnets are well matched to the armature poles field.

      For that to be true a few things have to happen. Design is first and foremost. Operating
design. A motor is normally run at a set voltage and designed for a set load. Who knows what
the FT-16D was in it's first life? Not me. But say it was a automotive door power window
actuator. In that instance the design would be for somewhere between 12 and 14.6 volts
centering 13.6 or so. It's "load" or current would have been for cast and the wind and magnet
package designed around that requirement.  Say 4 amps. Say at a four amp draw the motor
runs at 85% of no load speed for the 13.6 supply voltage. Say this produces 85% efficiency. So
now the design crew lights it up and finds that at 13.6 volts and 4 amps 15 degrees of timing
provide the "neutral" commutation plane and thus maximum motor life. They have pinpointed
all the pertinent design elements to operate for some predictable period of cycles of operation
at that load. Winding, magnet, timing etc all designed for that operating event.

      At this point the cheapest magnet possible has been selected that will match the strength
of 70 turns of 30 gauge at a four amp draw without invading the area of the magnets operating
curve in where is would be demagnetized but such field.  

      So a fella looks this motor over and says I can sell these as toy motors for scale cars at 10
cents on the dollar and in a slot car they go. There new operating environment subjects this
motor to a good deal more load than it is designed to handle. More voltage, heavier average
and peak current loads that create greater strength of armature pole now take the operating
point well past optimum leaving two glaring short comings. Not enough field for the pole load.
Demagnetisation of the permanent magnet field. How quickly? Oh...how hot a wind?

      When this happens Kirchoff falls apart. More field, for awhile, gives not only more torque
on less current (higher conversion) but MORE rpm as well!!!

                                              HP = (Torque * RPM) / 5252

      Look this formula for horsepower over. Look at it in all possible operating settings.

      In our second scenario where both RPM and Torque are on the rise then HP gains are
unavoidable as torque and rpm rise together. As we reach just enough something else takes
place. Torque continues "for a short range of rpm", to rise without a gain in RPM and you still
make more horsepower. This may be a few hundred rpm or perhaps a thousand but not huge
and in scale with the natural rpm limit of the motor. The higher she spins naturally the larger
the rpm gain in phase two of field strength. Lastly we get more field than we need. What
happens then?

      Kirchoff happens!! This is where "normal" motors are designed to operate in the "other
world" outside slots. How is that useful??


      Controlling grip and setting the speed limit. You don't have a gear change box so the
motor must cover in torque and rpm the range of "rate of work" that package is capable off. Of
course the motor has to make enough torque to do the task.

      Torque is how much work is getting done. Horsepower is how fast that work is done.

      ******************************************************************************************************

      Okay, what is Dynamic Timing? Armature reactance distorts the flux in the gap twisting it
in the direction of rotation. This distortion in distance and effect is dependant on load (current)
and rpm (time) and to a degree the amount of magnet field orientation.

      This means that the amount of timing is a correction to unwind that distortion so that the
armature field and magnet fields can meet at the correct 90 degree angle to maximize the
torque impulse. The fact that it is load and rpm sensitive means that timing on a drag motor is
only correct at ONE load and ONE rpm and the best place for that to happen is at stall
conditions. Zero rpm, Maximum current. That DOES not mean zero timing. Remember
electrical time constants? Has to be far enough in advance to allow for the number of degrees
of rotation for the amount of time it takes for "switch on" to "full saturation" in milliseconds of
the pole to place it in that 'sweet spot' of angularity.

      The more powerful the magnets resistance to that field shift the less timing the motor
needs. Neo's and Cobalt motor need less timing that parallel field ceramics. Segmented
ceramics or radials need more than parallel orientation is specific to the degree of timing.

      ****************************************************************************************************

      Okay, I'm going to cut the branch off behind me and give a road coarse example of timing
effects.

      Road motors on tracks that favor nothing more that a blip on the throttle to drive operate
pretty close to a steady speed and load. More so than a drag motor anyway and timing and it's
effects are more closely related to that "other world". The more you have to throttle the more it
behaves like a drag motor. You road guys have a much bigger basket of snakes. You operate
on lower and sometimes changing voltage and less current on spike and more over the
average. I can not imagine the number of arm timing experiments it would take to perfect a
"builders" class motor. Just say'n, then stay'n out of it.   

          
Horsepower (3)